Convincing people to use apps such as Signal is hard work and most can’t be convinced. But with those you manage to convince, do you feel happy to talk to them on Signal?
The problem is these people use Signal on Android/IOS which can’t be trusted and IOS has recently been in the news for having a backdoor. And it has also been revealed that american feds are able to read everyone’s push notifications and they do this as mass surveillance.
So not only do you have to convince people to use Signal which is an incredibly difficult challenge. You also have to convince them to go into settings to disable message and sender being included in the push notifications. And then there’s the big question is the Android and IOS operating systems are doing mass surveillance anyway. And many people find it taking a lot of effort to type on the phone so they install Signal on the computer which is a mac or Windows OS.
So I don’t think I feel comfortable sending messages in Signal but it’s better than Whatsapp.
These were some thoughts to get the discussion started and set the context.
There are several open protocols that meet your criteria that aren’t Matrix (with most of them using double-ratchet encryption similar to if not exactly like Signal). Due to server costs (Matrix eats a lot of RAM & storage), medium-sized entities usually bow out so the Matrix network largely consist of a few 1–10 user servers & massive centralization around Matrix.org & the hosted servers they provide. Since almost all the messages get synced to the Matrix.org server if just one Matrix.org user is in your room or whatever, Matrix have the largest access to all that metadata.